Low force level detection system and method

ABSTRACT

A method for estimating a force exerted by a first body onto a second body including the steps of providing a motor having at least one detectable motor signal, wherein the motor is adapted to advance the first body into engagement with the second body, determining a first value for the motor signal prior to the first body engaging the second body, determining a second value for the motor signal after the first body engages the second body, and generating a force value based upon a comparison of the second value to the first value.

The low force level detection system and method was created during theperformance of a cooperative research and development agreement with theDepartment of the Air Force (Contract No. F33615-03-2308 P00002).Therefore, the government of the United States may have certain rightsto the low force level detection system and method.

BACKGROUND

The low force level detection system and method relates to systems andmethods for detecting force and, more particularly, systems and methodsfor detecting low force levels in electromechanical brake systems.

Electromechanical brake systems typically include a housing, a rotor,brake pads, an actuator/caliper and a motor. The actuator is adapted todrive the brake pads into engagement with the rotor, thereby clampingthe rotor between the pads. The motor drives the actuator intoengagement with the brake pads and the rotor. Therefore, the amount offorce applied to the rotor by the brake pads is a function of thedistance that the actuator is advanced by the motor.

Prior art systems attempt to detect the position of the actuator at theonset of force (i.e., the point of initial contact). Then, assuming thatthe brake system can be modeled as a spring, the braking force may becalculated using Hooke's law:F ₀ =k(x ₀ −x _(i))  (Eq. 1)wherein F₀ is the braking force, k is the spring function of the system,x_(i) is the position of the actuator at the onset of force and x₀ isthe position of the actuator at a subsequent time.

Such prior art systems attempt to detect the actual contact point.However, at the onset of force, the signal to noise level of thedistinguishable signals is small and difficult to use. Therefore, suchsystems often encounter difficulty identifying the point at whichcontact is made and/or force is applied, thereby giving rise toinaccurate measurements of braking force.

Accordingly, there is a need for an improved system and method forestimating the contact force between two bodies.

SUMMARY

In one aspect, a method for estimating a force exerted by a first bodyonto a second body is provided. The method may include the steps ofproviding a motor having at least one detectable motor signal, whereinthe motor is adapted to advance the first body into engagement with thesecond body, determining a first value for the motor signal prior to thefirst body engaging the second body, determining a second value for themotor signal after the first body engages the second body, andgenerating a force value based upon a comparison of the second value tothe first value.

In another aspect, a brake system is provided. The brake system mayinclude a rotor, an actuator aligned to engage the rotor, a motoradapted to advance the actuator into engagement with the rotor, themotor generating at least one detectable motor signal, at least onesensor positioned to monitor the motor signal, and a processor incommunication with the sensor, the processor being adapted to determinea first value of the motor signal prior to the actuator engaging therotor and a second value of the motor signal after the actuator engagesthe rotor and generating a force value based upon a comparison of thesecond value to the first value.

In another aspect, a method for estimating a clamping force betweenbrake pads and a rotor is provided, wherein the brake pads are urgedinto engagement with the rotor by a motor having at least one detectablemotor signal. The method may include the steps of monitoring a motorsignal of the motor, the motor signal including at least one of a motorspeed, a motor current, a commutation time between motor position pulsesand estimates thereof, determining a first value for the motor signalprior to the brake pads engaging the rotor, determining a second valuefor the motor signal after the brake pads engage the rotor, anddetermining a clamping force value based upon a comparison of the secondvalue to the first value.

Other aspects of the low force level detection system and method willbecome apparent from the following description, the accompanyingdrawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of an electromechanical brake systemaccording to one aspect of the low force level detection system andmethod;

FIG. 1B is a schematic illustration of the electromechanical brakesystem of FIG. 1A with the actuator at a second position;

FIG. 1C is a schematic illustration of the electromechanical brakesystem FIG. 1A with the actuator at a third position; and

FIG. 2 is a graphical illustration of the low force level detectionsystem and method showing clamping force and current plotted againstposition of the actuator.

DETAILED DESCRIPTION

As shown in FIG. 1A, the disclosed low force level detection system andmethod is embodied in an electromechanical brake system, generallydesignated 10. Brake system 10 may include a housing 12, a motor 14(e.g., an electric motor), a processor 15, a sensor 17, an actuator 16,two brake pads 18, 20, a rotor 22. The motor 14 may include a ball screwassembly and a gear train (not shown) that may translate the rotationalforce of the motor 14 into distal advancement of the actuator 16,thereby urging the actuator 16 linearly into engagement with the brakepads 18, 20. As the actuator 16 engages the brake pads 18, 20, the brakepads 18, 20 clamp the rotor 22 and supply a braking force to the rotor22, as shown in FIG. 1B.

As shown in FIG. 2, as the actuator 16 moves from its initial positionP_(i) (see also FIG. 1A) to the clamping position P_(c) (see also FIG.1B), the motor 14 may be generally in a no-load state (i.e., there is noclamping force exerted by the motor 14). In the no-load state, theno-load motor speed ω_(NL) (motor speed is not shown in FIG. 2) and theno-load motor current I_(NL) may remain relatively constant when aconstant voltage is applied across the motor 14. However, as theactuator 16 contacts the brake pads 18, 20 and initiates clamping of therotor 22 (i.e., at position P_(c)), there may be a decrease in motorspeed ω and a corresponding increase in motor current I. FIG. 2graphically illustrates an example of the clamping force F relative tothe motor current I when a constant voltage is applied across the motor14, wherein there is a sudden increase in motor current I as theclamping force F begins to increase (i.e., beyond position P_(c)).

Accordingly, the following motor and actuator equations may be used tomodel the brake system 10:Jω=K _(T) I−T _(L) −μω−T _(C)  (Eq. 2)LI=V−RI−K _(E)ω  (Eq. 3)wherein J is the inertia of the motor 14, ball screw and gear train, ωis the motor speed, K_(T) is the motor torque constant, I is the motorcurrent, T_(L) is the load torque, μ is the dynamic friction in themotor 14, ball screw and gear train, T_(C) is the cogging torque, L isthe motor inductance, V is the source voltage, R is the motor resistanceand K_(E) is the EMF constant.

Assuming no load torque (i.e., T_(L)=0) when the motor 14 is in theno-load state, Eqs. 2 and 3 may be solved to yield: $\begin{matrix}{\omega_{NL} = \frac{{K_{T}V} - {RT}_{C}}{{\mu\quad R} + {K_{T}K_{E}}}} & \left( {{Eq}.\quad 4} \right) \\{I_{NL} = \frac{{\mu\quad V} + {K_{E}T_{C}}}{{\mu\quad R} + {K_{T}K_{E}}}} & \left( {{Eq}.\quad 5} \right)\end{matrix}$Then, assuming T_(L)=T_(F0) at a motor speed of ω_(F0) or a motorcurrent of I_(F0), Eqs. 2 and 3 may be solve to yield: $\begin{matrix}{\omega_{F\quad 0} = \frac{{K_{T}V} - {R\left( {T_{C} + T_{F\quad 0}} \right)}}{{\mu\quad R} + {K_{T}K_{E}}}} & \left( {{Eq}.\quad 6} \right) \\{I_{F\quad 0} = \frac{{\mu\quad V} + {K_{E}\left( {T_{C} + T_{F\quad 0}} \right)}}{{\mu\quad R} + {K_{T}K_{E}}}} & \left( {{Eq}.\quad 7} \right)\end{matrix}$Combining Eqs. 4 and 6 yields: $\begin{matrix}{\frac{\omega_{F\quad 0}}{\omega_{NL}} = {\frac{{K_{T}V} - {R\left( {T_{C} + T_{F\quad 0}} \right)}}{{K_{T}V} - {T_{C}R}} = {1 - \frac{T_{F\quad 0}R}{{K_{T}V} - {T_{C}R}}}}} & \left( {{Eq}.\quad 8} \right)\end{matrix}$and combining Eqs. 5 and 7 yields: $\begin{matrix}{\frac{I_{F\quad 0}}{I_{NL}} = {\frac{{\mu\quad V} + {K_{E}\left( {T_{C} + T_{F\quad 0}} \right)}}{{\mu\quad V} + {K_{E}T_{C}}} = {1 + \frac{K_{E}T_{F\quad 0}}{{\mu\quad V} + {K_{E}T_{C}}}}}} & \left( {{Eq}.\quad 9} \right)\end{matrix}$wherein ω_(F0) is the motor speed corresponding to clamping force F₀,I_(F0) is the motor current corresponding to clamping force F₀ andT_(F0) is the load torque at clamping force F₀.

Solving Eqs. 6 and 9 for T_(F0) yields the following equations:$\begin{matrix}{T_{F\quad 0} = {\frac{1 - \frac{\omega_{F\quad 0}}{\omega_{NL}}}{R}\left( {{K_{T}V} - {RT}_{C}} \right)}} & \left( {{Eq}.\quad 10} \right) \\{T_{F\quad 0} = {\frac{\frac{I_{F\quad 0}}{I_{NL}} - 1}{K_{E}}\left( {{\mu\quad V} + {K_{E}T_{C}}} \right)}} & \left( {{Eq}.\quad 11} \right)\end{matrix}$wherein, in one aspect, K_(T), μ, T_(C), V, R and K_(E) may be presumedto be relatively constant.

Thus, the load torque T_(F0) at clamping force F₀ may be determined bymeasuring the motor speed ω_(F0) relative to the no-load motor speedω_(NL) and using Eq. 10 or, alternatively, by measuring the motorcurrent I_(F0) relative to the no-load motor current I_(NL) and usingEq. 11.

Furthermore, the load torque T_(F0) may be related to the clamping forceF₀ as follows:T _(F0) =F ₀ G  (Eq. 12)wherein G is the gain. The gain G may be a function of the screw pitch,the gear reduction and/or the efficiency of the actuator. However, thegain G may be generally constant at relatively low clamping forces F₀.

Therefore, according to one aspect, the clamping force F₀ may bedetermined as follows: $\begin{matrix}{F_{0} = {\kappa_{1}\left( {1 - \frac{\omega_{F_{0}}}{\omega_{NL}}} \right)}} & \left( {{Eq}.\quad 13} \right) \\{or} & \quad \\{F_{0} = {\kappa_{2}\left( {\frac{I_{F\quad 0}}{I_{NL}} - 1} \right)}} & \left( {{Eq}.\quad 14} \right)\end{matrix}$wherein κ₁ and κ₂ are constants. In one aspect, constants κ₁ and/or κ₂may be determined graphically and/or by experimental data. In anotheraspect, constants κ₁ and/or κ₂ may be calculated by determining thevarious values of K_(T), μ, T_(C), V, R, K_(E) and G.

At this point those skilled in the art will appreciate that variousmotor signals may be used according to the low force level detectionsystem and method. For example, commutation time between motor positionpulses and estimates of motor signals may be used.

Accordingly, the clamping force F₀ applied to the rotor 22 by the brakepads 18, 20 and the actuator 16 may be determined by measuring a motorsignal value (e.g., motor speed or motor current) relative to the motorsignal value at a no-load state using the sensor 17 such that theprocessor 15 may correlate the measured value into a clamping forcevalue (i.e., a low force level).

In another aspect, the low force level detection system and method mayprovide a technique for estimating brake pad wear and/or the thicknessT_(X) of the brake pads 18, 20 at some subsequent time after use.

The brake system 10 may be provided with new or full brake pads 18, 20,wherein both brake pads 18, 20 and all linings have an initial thicknessT_(i). In one aspect, each individual brake pad 18, 20 may be presumedto have a thickness of about ½ of the total pad thickness T_(i) (i.e.,the initial thickness of each pad may be ½T_(i)). In another aspect,each brake pad 18, 20 may be presumed to wear generally equally.

Referring to FIGS. 1A and 1B, when the system 10 is provided with new orfull brake pads 18, 20, the actuator 16 may have an initial positionP_(i) (i.e., a fully retracted position) and a clamping position P_(C)(i.e., the position where the actuator 16 and brake pads 18, 20initially begin to clamp the rotor 22). The actuator may be moved to theinitial position P_(i) by fully backdriving the actuator 16.

The nominal distance D_(i) traveled by the actuator 16 from the initialposition P_(i) to the clamping position P_(C) when the brake pads 18, 20are new or full may be determined as follows:D _(i) =P _(C) −P _(i)  (Eq. 15)

As shown in FIG. 1C, after use and associated wear of the brake pads 18,20, the actuator 16 must travel to position P_(X) to initiate clamping.Accordingly, the thickness T_(X) of the brake pads 18, 20 at somesubsequent time after use may be determined as follows:T _(X) =T _(i)−[(P _(X) −P _(i))−D _(i)]  (Eq. 16)wherein, the total pad wear may be determined as follows:Total Pad Wear=T _(i) −T _(X)  (Eq. 17)In one aspect, each pad 18, 20 may be presumed to have a thickness ofabout ½T_(X) at some subsequent time after use.

The positions P_(C), P_(X) of the actuator 16 at the onset of clampingmay be determined using any known techniques. In one aspect, positionsP_(C), P_(X) of the actuator 16 may be determined by monitoring motorsignals, as discussed above. However, those skilled in the art willappreciate that any technique capable of determining positions P_(C),P_(X) may be used.

For example, the distance between the fully backdriven position of anactuator and the onset of clamping of a new pair of brake pads (totalthickness of 20 mm) may be about 30 mm. After several months of use, thedistance between the fully backdriven position of the actuator and theonset of clamping may be about 34 mm. Therefore, applying Eq. 16, theresulting pad thickness may be estimated to be about 16 mm (i.e., 20mm−[(34 mm)−(30 mm)]), wherein each brake pad may be about 8 mm thick.

Although the low force level detection system and method is shown anddescribed with respect to certain aspects, modifications may occur tothose skilled in the art upon reading the specification. The low forcelevel detection system and method includes all such modifications and islimited only by the scope of the claims.

1. A method for estimating a force exerted by a first body onto a secondbody comprising the steps of: providing a motor having at least onedetectable motor signal, wherein said motor is connected to said firstbody to advance said first body into engagement with said second body;determining a first value for said motor signal representative of aconfiguration in which said first body does not engage said second body;determining a second value for said motor signal, different from saidfirst value, representative of a second configuration in which saidfirst body does engage said second body; and generating a force valuebased upon a comparison of said second value to said first value.
 2. Themethod of claim 1 wherein said motor is an electric motor.
 3. The methodof claim 1 wherein said detectable motor signal includes at least one ofmotor speed, motor current, commutation time between motor positionpulses and estimates thereof.
 4. The method of claim 1 wherein saidgenerating step further includes multiplying said comparison by aconstant.
 5. The method of claim 1 wherein said force value is aclamping force.
 6. The method of claim 1 wherein said first value isbased upon a generally no-load state of said motor.
 7. The method ofclaim 1 wherein said first value is generally constant when said firstbody is not engaging said second body.
 8. The method of claim 1 whereinsaid motor signal is motor speed and said motor speed decreases aftersaid first body engages said second body.
 9. The method of claim 1wherein said motor signal is proportional to motor current and saidmotor current increases in response to said first body engaging saidsecond body.
 10. The method of claim 1 wherein said first body is anactuator and said second body includes a rotor and at least one brakepad.
 11. The method of claim 10 further comprising the step ofestimating the thickness of said brake pad based at least upon anoriginal thickness of said brake pad and a distance traveled by saidactuator.
 12. A brake system comprising: a rotor; an actuator positionedto engage said rotor; a motor adapted to advance said actuator intoselective engagement with said rotor, said motor generating at least onedetectable motor signal representative of an operating condition of saidmotor; at least one sensor positioned to monitor said motor signal; anda processor connected to said sensor, said processor being adapted todetermine a first value of said motor signal representative of aconfiguration in which said actuator does not engage said rotor and asecond value of said motor signal representative of a configuration inwhich said actuator does engage said rotor and generating a force valuebased upon a comparison of said second value to said first value. 13.The system of claim 12 wherein said processor includes said sensor. 14.The system of claim 12 wherein said motor is an electric motor.
 15. Thesystem of claim 12 wherein said detectable motor signal includes atleast one of motor speed, motor current, commutation time between motorposition pulses and estimates thereof.
 16. The system of claim 12further comprising at least one brake pad positioned between saidactuator and said rotor.
 17. The system of claim 12 wherein said sensoris a motor speed sensor and said detectable signal is motor speed. 18.The system of claim 12 wherein said sensor is a motor current sensor andsaid detectable signal is motor current.
 19. A method for estimating aclamping force between brake pads and a rotor, said brake pads beingurged into engagement with said rotor by a motor having at least onedetectable motor signal, said method comprising the steps of: monitoringa motor signal of said motor, said motor signal including at least oneof a motor speed, a motor current, a commutation time between motorposition pulses and estimates thereof; determining a first value forsaid motor signal representative of a condition in which said brake padsdo not engage said rotor; determining a second value for said motorsignal representative of a condition in which said brake pads engagesaid rotor; and determining a clamping force value based upon acomparison of said second value to said first value.
 20. The method ofclaim 19 further comprising the step of determining a pad wear value ofsaid brake pads, wherein said brake pads have an initial thickness and acurrent thickness, said step including: providing an actuator movable bysaid motor from a fully retracted position to a clamped position;determining a first distance between said fully retracted position andsaid clamped position when said brake pads are at said initialthickness; determining a second distance between said fully retractedposition and said clamped position when said brake pads are at saidcurrent thickness; and comparing said first distance to said seconddistance;